diff --git a/dom/fetch/Fetch.cpp b/dom/fetch/Fetch.cpp
index 39b31065c5fb..5f192f72aafa 100644
--- a/dom/fetch/Fetch.cpp
+++ b/dom/fetch/Fetch.cpp
@@ -451,7 +451,7 @@ ExtractFromURLSearchParams(const URLSearchParams& aParams,
   nsAutoString serialized;
   aParams.Stringify(serialized);
   aContentType = NS_LITERAL_CSTRING("application/x-www-form-urlencoded;charset=UTF-8");
-  return NS_NewStringInputStream(aStream, serialized);
+  return NS_NewCStringInputStream(aStream, NS_ConvertUTF16toUTF8(serialized));
 }
 } // namespace
 
diff --git a/dom/workers/ServiceWorkerScriptCache.cpp b/dom/workers/ServiceWorkerScriptCache.cpp
index fa25947d4959..5ffec43aa2db 100644
--- a/dom/workers/ServiceWorkerScriptCache.cpp
+++ b/dom/workers/ServiceWorkerScriptCache.cpp
@@ -537,7 +537,8 @@ private:
 
     ErrorResult result;
     nsCOMPtr<nsIInputStream> body;
-    result = NS_NewStringInputStream(getter_AddRefs(body), mCN->Buffer());
+    result = NS_NewCStringInputStream(getter_AddRefs(body),
+                                      NS_ConvertUTF16toUTF8(mCN->Buffer()));
     if (NS_WARN_IF(result.Failed())) {
       MOZ_ASSERT(!result.IsErrorWithMessage());
       Fail(result.StealNSResult());